1. Technical Field
The disclosure relates to a hole-assisted optical fiber.
2. Related Art
Conventionally, an optical fiber compliant with ITU-T G.652 (made by International Telecommunication Union) has been used for optical communications. In the ITU-T G.652, as optical fiber characteristics, a mode field diameter at a wavelength of 1310 nm is specified as in a range of 8.6 to 9.5 (±0.6) μm, a cut-off wavelength by a 22 m method defined in ITU-T G.650 is specified as equal to or less than 1260 nm, and a zero-dispersion wavelength is specified as in a range of 1300 to 1324 nm.
A hole-assisted optical fiber (HAF) is an optical fiber having a constitution such that holes are provided in a cladding portion formed around a core portion to which germanium or the like is added, the core being high in refraction index (see Japanese Patent No. 3854627 and Japanese Patent Application Laid-open No. 2004-226540). The hole-assisted optical fiber is provided with the holes to intensify the confinement of light to the core portion thus being characterized by a decrease in bending loss. Therefore, bending loss characteristics further more excellent than that specified in the ITU-T G.657B can be realized and hence, the hole-assisted optical fiber has come to attention as an optical fiber used in place of the above-mentioned conventional optical fiber compliant with the ITU-T G.652.
In addition, in the ITU-T G.652, the value of the bending loss when bent at a radius of 7.5 mm is specified as equal to or less than 1 dB/turn for a wavelength of 1625 nm and equal to or less than 0.5 dB/turn for a wavelength of 1550 nm. Here, “dB/turn” is a unit expressing the increase in transmission loss per one turn in decibels when an optical fiber is wound by one around (one turn) at a predetermined radius.
Here, when the hole-assisted optical fiber is used as an optical communications-use optical fiber compliant with ITU-T G.652, it is necessary to realize single-mode transmission at a wavelength (1556 nm, for example) used for optical communications. Furthermore, at the same time, in order to improve productivity, the hole-assisted optical fiber preferably has a large design margin for achieving desired properties thereof.